A.T. Yokobori

Analysis of damage behaviour based on EBSD method under creep–fatigue conditions for polycrystalline nickel base superalloys

Abstract In order to detect creep–fatigue damage before the crack initiation stage, an investigation into damage behaviour based on the electron backscattered diffraction (EBSD) method for a polycrystalline nickel base superalloy has been carried out. The average misorientation within grains increases linearly up to the initiation of cracks with the increase in creep strains. Even if stress distributions exist by stress concentration, assessment in all the damage areas allowed the evaluation of creep damage regardless of geometrical influence. Furthermore, the influence of strain rates, introducing fatigue and testing temperatures are hardly observed in the misorientation analysis. Misorientation almost corresponded to inelastic strain regardless of those influences. Quite a small misorientation caused by fatigue led to an equivalent result between creep conditions and creep–fatigue conditions. It is concluded that the misorientation analysis of damaged materials based on the EBSD method allows the quantitative estimation of creep strain and the assessment of remaining creep fracture life.

Micro- and macro-creep damage formation for P92 under multiaxial stress related to circular notched specimen

Abstract In this study, to clarify the behaviour of micro- and macro-creep damage progression for P92 under multiaxial stress field, interrupted creep tests, analysis of multiaxial stress and detailed the cross-sectional observations were conducted on a circular notched round bar specimen which produces the multiaxial stress field due to the plastic constraint. As a result, creep voids were initiated at the early stage and they were formed up to the final fracture. These phenomena were found to be detected using direct current potential drop (DCPD) method. These results concern the development of the measurement of creep crack initiation. The distribution of high void area fraction was in good agreement with that of high hydrostatic stress and high multiaxial stress. This result indicates that multiaxial stress affects the void formation. Furthermore, the micro-creep damage of each interrupted specimen was evaluated by using the electron backscatter diffraction (EBSD) method which can analyse crystallographic misorientation caused by creep strain. The results of EBSD analyses indicated that the value of grain reference orientation deviation (GROD) closely concerns the void initiation.

Law of Crack Growth Life for P92 Steel Under the Conditions of Creep-Fatigue Multiplication and Projection of the Threshold Temperature of Creep Crack Growth Based on This Law

W-added 9Cr ferritic heat-resistant steels, ASME grade P92, are used under the high temperature creep-fatigue condition. Concerning the maintenance of the operational safety and minimizing operational costs, it is necessary to construct a law of predicting the crack growth life by clarifying the dominant factors of the crack growth life under the condition of creep-fatigue multiplication (the effect of stress holding time on the time-dependent fracture). In this study, crack growth tests under the conditions of creep-fatigue multiplication were conducted with various conditions of stress holding time, applied stress and temperatures. As a result, a unified law of predicting the crack growth life under the conditions of creep-fatigue multiplication was derived based on the concept of Q* with the transition function of crack growth life from fatigue to creep.© 2009 ASME

Theoretical Representations of a Characteristic Curved Surface for the Life of IN738LC Under High Temperature Creep and Fatigue Interaction Based on Non-Equilibrium Science

The Ni-base supperalloy IN738LC, developed as a gas turbine blade material, is used under the conditions of creep-fatigue multiplication. In this paper, using IN738LC, in situ observational tests under the conditions of creep-fatigue multiplication were conducted and the effects of cycle-dependent and time-dependent mechanisms on the fracture life tf were investigated. Furthermore, on the basis of the concept of non-equilibrium science, the multiple effects of creep and fatigue on the fracture life tf were clarified.© 2009 ASME

Characteristic of Crack Growth Life for W Added 9Cr Ferritic Heat Resistant Steel Under the Conditions of High Temperature Creep-Fatigue Multiplication Based on Non-Equilibrium Science

The W added 9Cr ferritic heat resistant steel ASME grade P92, developed as a boiler tube material, is used under the conditions of creep-fatigue multiplication. In this paper, using P92 steel, crack growth tests under the conditions of creep-fatigue multiplication were conducted and the effects of cycle-dependent and time-dependent mechanisms on the crack growth life tf were investigated. Furthermore, on the basis of the concept of non-equilibrium science, the multiple effects of creep and fatigue on the crack growth life tf were clarified.Copyright

The Occurrence Mechanism of Periodicity of Creep Crack Path for P92

The morphology of creep crack growth in metals is usually dominated by inter-granular cracking. However, for developed heat-resistant steels with a martensitic lath strengthening structure such as ASME Grade P92, inter-granular cracking, which is a typical fracture mechanism of creep crack growth, was restrained, and the creep crack growth path was found to be composed of a fracture unit area (FA) beyond the scale of the grain size. In spite of the fact that FA is closely related to resistance against crack growth, the occurrence mechanism of FA has not yet been clarified. In this study, by conducting creep crack growth tests for P92, two-dimensional finite element analyses, and metallurgical characterization of the microstructure, the occurrence mechanism of FA for P92 was clarified. As a result, the creep crack growth path for P92 was found to be composed of an FA beyond the scale of the grain size due to the sub-critical crack growth, and the periodic sub-critical crack growth was found to be dominated by the preceding initiation of voids along a slip line. On the basis of the proposed finite element method analysis, this crack growth mechanism was found to cause dispersion of creep damage, which causes unstable crack growth and results in periodic convexo-concave crack growth, that is, FA-cracking.

Morphology of the creep crack tip in P92 steel and its relation to microstructure

The region around the tip of a creep crack obtained using an interrupted test on a C(T) specimen of the 9 wt.% Cr power plant alloy P92 has been examined using electron backscatter diffraction to study the relationship between the crack morphology and the martensitic substructure. Nucleation of voids was found to occur primarily on prior austenite grain and packet boundaries. This agrees well with previous results in which minimum creep rate decreased with increasing prior austenite grain size for small prior austenite grains. Growth and coalescence of voids to form a continuous crack appears to occur by recovery, recrystallisation and eventual rupture of the bridging regions between the voids.

Evaluation of creep crack growth properties of Gr. 92 steel weldment

High Cr ferritic heat resistant steels are used for boiler components in ultra-super critical thermal power plants. In weld components of these steels, Type-IV creep damages formed in the fine-grained heat-affected zone (HAZ), causing their creep strength to decrease at high temperatures. To assist the standardization of the testing method for creep crack growth in weld components, we conducted round robin tests using ASME Grade 92 steel as part of the VAMAS TWA31 collaboration. The CCG tests were carried out using the CT specimen and the circumferentially notched round bar specimen for both the base metal and weld joint of Gr. 92 steel. The effects of specimen configuration, temperature, load and stress triaxiality conditions on the crack initiation and growth properties were investigated.

Evaluation of damage and fracture of high Cr steel welds at elevated temperatures

High Cr ferritic steels have been used in the 600°C class ultra-super critical (USC) thermal power plants. More than a decade has passed from the application of high Cr steels to USC power plants in Japan, and type-IV creep damages in the welded components become concerns. In the present paper, long-term creep tests for the welded joints of the high Cr steels (Gr.91 and Gr.122 steels) were conducted. Microstructures and creep damages in the heat affected zone (HAZ) were investigated using the thick plate specimens. Remaining life assessment methods of high Cr steel welds were discussed based on the experimental results. Further, to assist the standardization of the testing method for creep crack growth in welded joints, we have conducted the round robin tests using the high Cr steels (Gr.91 and Gr.92 steels) and their welds as part of the VAMAS TWA31 collaboration.

The Effect of Load Frequency on the Temperature Dependence of Fracture Life of Notched Specimens for 9-12Cr Steel Under Creep-Fatigue Conditions

In order to operate a thermal power plant safely, it is necessary to establish the prediction methods of fracture life for heat-resistant steels which is used in thermal power plant. For the establishment of prediction methods, it is necessary to consider the effects of creep damage and creep-fatigue interaction on the fracture life.Therefore, in this paper, creep-fatigue tests were conducted on notched specimens of W-added 9-12Cr steels for various temperatures and load frequencies. From these results, the influence of load frequency on temperature dependence of crack growth life for 9-12Cr steels was determined.Copyright